Light emitting diodes (LEDs) are one type of semiconductor devices which emit optical radiation. LEDs are manufactured for a wide variety of applications with a wide range of radiometric, photometric and colorimetric characteristics.
In order to properly characterize the wide variety of LEDs and to properly compare different LEDs, efforts have been made to define standards for determining their various characteristics. One such effort has been undertaken by the Commission International de l'Eclairage (CIE) and has been published as Technical Report 127 (hereinafter “CIE 127”).
An important characteristic of LEDs that CIE 127 addresses is LED intensity. LEDs by their nature are directional light sources. That is, LEDs emit light in a narrow beam which is often broadened by diffusers or lenses. Near-field and far-field light measurements can yield quite varying results depending upon what detector areas are used and the distances between the detectors and the LEDs. Because different laboratories, users and manufacturers seldom agree on measurements of the same devices, the CIE has proposed a standard measurement technique that can be used to provide consistent results.
The measurement technique proposed in CIE 127 entails measuring the flux incident on a circular detector area of 100 mm2 at distances of 100 mm and 316 mm from the LED under test. FIG. 1A shows standard measurement condition A and FIG. 1B shows standard measurement condition B. To ensure a standard circular detector area of 100 mm2, a circular aperture 25 having an area of 100 mm2 is arranged in front of the detector 20. (Note that FIGS. 1A and 1B are not to scale.) In condition A the aperture 25 is 316 mm from the LED 10 and in condition B the aperture 25 is 100 mm from the LED 10. The distance between the aperture 25 and the detector 20 is not critical so long as all of the light that passes through the aperture is detected by the detector 20.
Condition A corresponds to a solid angle of view of 0.001 steradian, which corresponds to an equivalent plane angle of approximately 2 degrees, and Condition B corresponds to a solid angle of view of 0.01 steradian, which corresponds to an equivalent plane angle of approximately 6.5 degrees. The measurements are referred to as Averaged LED Intensity and designated by the symbols ILEDA and ILEDB, for the respective conditions. Each of these values is calculated as follows:ILED=E×d2,  (Eq. 1)where E is the average illuminance measured at the detector and d is the distance between the LED and the detector (i.e., 100 mm or 316 mm).
To comply with the requirements of CIE 127, conventional measurement arrangements require the movement of either or both the LED 10 and the detector 20 in order to perform the necessary measurements at the two distances. A typical arrangement uses some sort of optical rail or bench which holds the LED and detector in alignment and allows one or both to be moved. The rail or bench must be contained within a light tight enclosure and internally baffled to prevent detection of reflected or scattered light from the walls of the enclosure under both distance conditions.
Because either or both of the LED and detector must be moved and suitable baffling maintained, such conventional instrumentation becomes bulky and cumbersome to operate for bench top measurements.
The present invention provides an arrangement that overcomes the aforementioned shortcomings of the known art.